So, there was some speculation earlier about what Bartolomeo's devil fruit was (if he has one), that being mainly some zoan fruit, but based off of how he almost liquefied Hack's hand without even zipping up and turning around, I'm guessing it's not zoan.
If he does indeed have a devil fruit, then it's most likely a paramecia. Based off of what it did to Hack, it was either benign, and it was the force of Hack's own attack that caused his hand to get crushed and bloodied upon impact, or it was an active force, both protecting Barto and slicing up Hack's hand. So, based off of that, I thought of a couple different fruits that could explain it...
1. Force-field fruit, not only protecting the user but possibly sending the damage right back into the attacker.
2. Particle manipulation fruit, allowing him to control particles and maybe even atoms in the air, manipulating their speeds to the point where it can form a cutting like force.
3. Repel fruit or pulse fruit, allowing the user to create some kind of pulse or repelling force to emanate from his body, and doubles as a means of protection. (really unlikely)
What do you guys think? You can vote below or comment if you have some other idea.